Conventional honeycomb structures are usually formed by corrugating a ribbon of core material such that the opposite longitudinal edges define truncated triangular wave forms when viewed from the respective sides. Thus, there is a rising section, a horizontal section, a declining section and another horizontal section and then the pattern is repeated.
When successive ribbons are placed one on top of the other, there are defined six sided honeycombs, the lower horizontal portions of one ribbon engaging the upper horizontal portions of the ribbon beneath it, these engaging portions being termed the "nodes" of the honeycomb structure. The nodes themselves are welded together with brazing material so that a fairly large sheet of the honeycomb structural material can be provided and will hold together.
Panels may be provided on either side of the honeycomb sheet formed as above and brazed thereto so that a honeycomb panel results. In other instances, a honeycomb structure may be curved into a cylindrical shape or collar shape and an outer panel or skin only applied to the exterior. Such cylindrical shaped honeycomb structures are used widely in the aircraft industry since they provide a very high strength-to-weight ratio.
When a sheet or large area of honeycomb structural material is curved into a cylindrical shape, the one side which will constitute the outer wall of the cylinder is stretched while the opposite side constituting the inner wall of the cyliner is compressed. Thus, the truncated triangular wave forms followed by the opposite longitudinal edges are in turn respectively stretched and compressed. This action results in a change in the height of the truncated triangular wave form, the stretched longitudinal edges resulting in a decrease in the overall height of the wave form, while compression of a longitudinal edge increases the height of the wave form.
As a consequence of the foregoing action, the inner wall of the cylinder will have a height that is greater than the outer wall of the cylinder so that the entire cylinder is distorted into an hourglass-shape; that is, the central portion of the cylinder tends to bow inwardly.
While this bowing or deformation is slight, particularly in the case of large radii cylinders, in an attempt to provide for a straight cylindrical construction, strains are placed on the individual honeycomb cells and as a consequence some of the welded nodes can fail. Also, where an outer skin has been attached, the described deformation can result in failure of various weld points of the outer skin to the honeycomb structure.